Children with brain tumors are generally followed for response or progression by imaging studies, such as CT or MRI. While these imaging studies help delineate the anatomical location and extent of a tumor within the CNS, they give no information regarding the biologic or metabolic activity of the lesion. Proton Nuclear Magnetic Resonance Spectroscopic Imaging (1H-MRSI) is a non-invasive method of detecting and measuring cellular metabolites in vivo, providing biochemical information in conjunction with the spatial information obtained by MRI. Positron Emission Tomography (PET) is a technique that also provides data on metabolic activity of brain lesions. However, a comparison of these two methods in determining a lesion's metabolic activity has not been reported in children with brain tumors. This study will be conducted to compare biologic or metabolic activity of brain tumors in pediatric patients as determined by 1H-MRSI and [18F-]-FDG PET scanning and to correlate results of 1H-MRSI and 18F-FDG PET imaging with outcome.